Vibrators for generating tactile alerts in portable communications receivers are well known. Early devices comprised a motor driven offset mass for generating the tactile alert. Disadvantageously, such devices tended to have short lifetimes due to wear on bearings, commutators, brushes, etc. Also, when the portable communications receiver was worn on a person's body, the motor driven tactile vibrator generated movement not only in a direction useful for producing a tactile response, e.g., normal to the body, but also in other less useful directions, e.g., parallel to the body. As a result, such vibrators disadvantageously consumed large amounts of battery power for the amount of tactile response the vibrators produced.
As an improvement over the motor driven tactile vibrator, a resonant armature tactile vibrator has been developed that uses a movable mass suspended by a single planar spring suspension element and incorporates an axially polarized permanent magnet driven by an electromagnetic means to effect a vibration in a fundamental mode. This conventional resonant armature tactile vibrator overcomes many of the problems of the motor driven tactile vibrator, but has attendant limitations of its own. One such limitation is that, for mechanical clearance reasons during operation, the amount of vibrating mass that can be suspended by the planar spring suspension element for a given device volume is relatively small, thus requiring a relatively large device to produce a sufficiently strong tactile vibration. Another limitation is that the range of possible resonant frequencies is restricted by the thickness and displacement relationships of the single planar spring suspension element.
A further limitation to the performance of the conventional resonant armature tactile vibrator results from a critical coupling of the fundamental mode of vibration to other, spurious modes of vibration. The critical coupling exists because the design using a movable mass suspended by a single planar spring suspension element exhibits a torsional (second mode) resonant frequency that is very close to the resonant frequency of the fundamental mode. The second mode vibration that results from the critical coupling reduces the amplitude of the desired fundamental mode vibration and generates tri-axial stresses in the suspension element, greatly reducing the life cycle yield before failure of the device.
Still another limitation of the conventional resonant armature device is caused by the axial polarization of the permanent magnet interacting with magnetic shielding required around the device for protection of sensitive circuits in portable communications receivers. This interaction further reduces the amplitude of the desired fundamental mode vibration.
Thus, what is needed is a vibrator that retains the advantages of the conventional resonant armature tactile vibrator over the motor driven tactile vibrator, but overcomes the limitations of the conventional resonant armature tactile vibrator. More specifically, a vibrator that provides a greater vibrating mass for producing a greater tactile response within a given device volume is needed. In addition, a vibrator that can be manufactured to operate over a wide range of predetermined resonant frequencies without reducing life cycle yield is desired. Also, a vibrator that can decouple the desired fundamental mode of vibration from energy-robbing, life-cycle-reducing spurious modes of vibration is highly desired. A vibrator that can be magnetically shielded without significant detrimental interaction between the magnetic shield and the vibrating elements is needed.